1. Field of the Invention
The present invention relates to an apparatus and method for monitoring the depth of anaesthesia.
2. Related Art
Monitoring the depth of anaesthesia is a well-known objective as indicated by the pamphlet "Anaesthesia Rounds" No. 21 issued by ICI Pharmaceuticals, Alderley Park, Macclesfield, Cheshire, England in 1988. This publication summarises various techniques which have been proposed for monitoring the depth of anaesthesia. Interest in this subject has increased in recent years as the result of growing evidence that in a small but not negligible number of cases, patients are aware of their surroundings during anaesthesia. It is probable that the problem of awareness under anaesthesia has increased due to the availability of anaesthetics from which recovery is relatively rapid. The above publication outlines a wide range of possible methods of monitoring depth of anaesthesia which are considered to merit further study. These include electroencephalogram (EEG) analysis, auditory evoked responses (AER), autonomic signs, surface electromyogram (EMG), and oesophageal contractility.
The investigation of the depth of anaesthesia by using the various techniques mentioned above has enabled a body of data to be assembled which can indicate to anaesthetists appropriate anaesthetic techniques whereby different categories of patient can be anaesthetized reliably in most circumstances. This body of data is not sufficient, however, to cover all the circumstances and it is therefore necessary for the anaesthetist to maintain a very close watch on a patient during, for example, a surgical procedure. Given that modern anaesthetics have recovery periods of only two or three minutes in some circumstances, it is necessary for the anaesthetist to very closely monitor a patient almost continuously. Ideally a system should be provided which can give a real time indication of the depth of anaesthesia, but no such system has been made available.
The article "An Improved Method of Measuring Heart-rate Variability: Assessment of Cardiac Autonomic Function", Biometrics 40,855-861, September 1984 by C. R. Weinberg and M. A. Pfeifer, describes various techniques for assessing sinus arrhythmia using the R--R interval, that is the interval between the easily-distinguished spikes in an electrocardiogram (ECG). This parameter is used, for example, to distinguish between diabetics and others. The article notes that problems arise in relying simply on the R--R interval. For example, the standard deviation of the R--R interval varies with respiratory rate. It is suggested that the patient should maintain a fixed frequency of respiration. A new measure of sinus arrhythmia is suggested, based on the application of circular statistics, but it appears that a fixed rated of respiration is still required. There is no suggestion that the derived information could be used in real time to monitor the depth of anaesthesia, the technique being put forward essentially with the purpose of enabling better identification of diabetics.
In the article "Respiratory Sinus Arrhythmia During Recovery From Isoflurane-nitrous-oxide Anaesthesia), Anesth. Analg. 1985; 64:811-15, by Y. Donchin, J. M. Feld, and S. W. Porges, it is suggested that on-line analysis of respiratory sinus arrhythmia, provides a physiological index of the depth of anaesthesia and the rate of recovery from anaesthesia. The measure proposes monitoring the variance of the heart rate pattern in the frequency band of respirations, this measure being indicative of the depth of anaesthesia. An arbitrary limit is suggested against which this measure is compared to determine the depth of anaesthesia and thus the reference limit is not patient specific. As it is not patient specific, it is unlikely that anaesthetists could rely upon this method to give real time indications of a need to increase the supply of anaesthetic to a patient to prevent premature recovery. This is because different patient exhibit different sinus arrhythmia responses, when anaesthetized. For example, in the extreme case of diabetics no response is detectable.
In the article "RR Variation: The Autonomic Test of Choice in Diabetes". Diabetes/Metabolism Reviews, Vol. 4, No. 3, 255-271 (1988), by H. Genovely and N. A. Pfeifer, further work related to the use of sinus arrhythmia in the diagnosis of diabetics is described. This article elaborates the application of circular statistics to derive a measure of sinus arrhythmia. It is also suggested that the periodicity of the unit circle used in the circular statistical analysis may be changed to match variations in respiratory rate in, for example, children and animals where the "patient" cannot be relied upon to co-operate by breathing at a regular rate. It describes the significance of "clustering" in circular statistics, the degree of clustering being identified by the length of a vector. The greater the length of the vector, the larger is the R--R variation and hence the sinus arrhythmia. Thus techniques are known which enable a measure of sinus arrhythmia to be derived but not to correlate measurements of sinus arrhythmia with the depth of anaesthesia or the rate of recovery of a specific patient.